The goal of this project is to further characterize the actions of 1,25(OH)2D3, both in vivo and at the molecular level. In vivo investigations performed in vitamin D-deficient animals have revealed that this hormone plays a key role in the regulation of intestinal calcium absorption. To better understand the role of both the receptor-mediated and "non-genomic" effects of 1,25(OH)2D3, the research proposed is directed toward designing a genetically engineered mouse lacking a functional 1,25(OH)2D3 receptor. The model will permit identification of function of 1;25(OH)2D3 from embryogenesis to senescence, and will allow the characterization of effects of this hormone that are truly receptor- independent. In the proposed model, the heterozygous mother is expected to be phenotypically normal and normocalcemic, and normal litermates will act as controls, thus, the effects of 1,25(OH)2D3 receptor deficiency on embryonic development will be addressed. The human disease, Vitamin D Rickets Type II (VDDR II), characterized by a homozygous mutation of the 1,25(OH)2D3 receptor, suggests that an animal model of 1,25(OH)2D3 receptor deficiency will provide highly useful information. To address the actions of 1,25(OH)2D3 at a molecular level, the transcriptional downregulation by 1,25(OH)2D3 of the gene encoding human parathyroid hormone will be investigated. Cellular factors, other than the 1,25(OH)2D3 receptor, are necessary for downregulation of human parathyroid hormone (hPTH) fusion genes. Identification and characterization of the factors that interact with the 1,25(OH)2D3 receptor to mediate the transcriptional repression of the hPTH gene may further our understanding of secondary hyperparathyroidism associated with chronic renal failure.